Preparation of membranes using solvent-less vapor deposition followed by in-situ polymerization

ABSTRACT

A system of fabricating a composite membrane from a membrane substrate using solvent-less vapor deposition followed by in-situ polymerization. A first monomer and a second monomer are directed into a mixing chamber in a deposition chamber. The first monomer and the second monomer are mixed in the mixing chamber providing a mixed first monomer and second monomer. The mixed first monomer and second monomer are solvent-less vapor deposited onto the membrane substrate in the deposition chamber. The membrane substrate and the mixed first monomer and second monomer are heated to produce in-situ polymerization and provide the composite membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention is a continuation-in-part of U.S. patent application Ser.No. 11/486,669 filed Jul. 14, 2006, now issued U.S. Pat. No. 7,754,281,which claims benefit of U.S. provisional application Ser. No. 60/700,650filed Jul. 18, 2005.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND

1. Field of Endeavor

The present invention relates to membranes and more particularly topreparation of membranes using solvent-less vapor deposition followed byin-situ polymerization.

2. State of Technology

U.S. Pat. No. 5,817,165 for fluorine-containing polyimide gas separationmembrane and method of manufacturing the same issued Oct. 6, 1998 toHisao Hachisuka et al provides the following state of technologyinformation: “Polyimide is known as a gas separation membrane materialwith excellent heat-resisting and anti-chemical properties due to itshigh glass transition point and rigid molecular chain structure. Themanufacture of a thinner and more asymmetric separation membrane havinga mechanical strength suitable for practical usage has been considered.When a polymer having a high separation factor is formed as a membraneon a proper porous supporting film, the membrane preferably should be0.1 μm thick or less to obtain a practical permeability. As a result,the manufacturing process becomes complicated, the yield deterioratesand the cost is raised, and thus it is impractical for industrial use.”

U.S. Pat. No. 6,497,747 for production and use of improved polyimideseparation membranes issued Dec. 23, 2002 to Yong Ding et al providesthe following state of technology information: “The use of polymericmembranes for gas separation applications is well documented in the art.The relationship between the polymeric structure and the gas separationproperties has been extensively studied, see for example, W. J. Koros,Journal of Membrane Science, Volume 83, ppl, 1993; L. M. Robeson,Journal of Membrane Science, Volume 62, pp 165, 1991; and L. M. Robeson,Polymer, Volume 35, pp 4970, 1994. It is well documented in the art thatstiffening the polymeric backbone while simultaneously inhibiting chainpacking can lead to improved gas permeability combined with an increasein gas selectivity for certain gas mixtures. Polyimides are examples ofsuch rigid-rod polymers showing desirable gas separation properties, seefor example, D. R. B. Walker and W. J. Koros, Journal of MembraneScience, Volume 55, p 99, 1991; S. A. Stern, Journal of MembraneScience, Volume 94, p 1, 1994; K. Matsumoto, P. Xu, Journal of AppliedPolymer Science, Volume 47, p 1961, 1993. U.S. Pat. Nos. 4,705,540;4,717,393; 4,717,394; 5,042,993; and 5,074,891 disclose the preparationof such aromatic polyimide gas separation membranes. For practicalindustrial applications polymeric gas separation membranes arefabricated into an asymmetric or a composite configuration with thinseparation layers. The membranes can be further configured into flatsheets or into hollow fibers. Although rigid-rod polyimides haveexcellent gas separation properties, they frequently can be dissolvedonly in aggressive organic solvents such as N-methyl-pyrrolidinone(NMP), N,N-dimethyl formamide (DMF), or phenols which makes it difficultto prepare composite membranes with ultrathin separation layers and canfurther cause environmental problems. For example, polyimide membraneshave been fabricated from chlorophenol solutions as described in U.S.Pat. No. 4,440,643.”

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides a system of fabricating a compositemembrane from a membrane substrate using solvent-less vapor depositionfollowed by in-situ polymerization. A first monomer and a second monomerare directed into a mixing chamber in a deposition chamber. The firstmonomer and the second monomer are mixed in the mixing chamber providinga mixed first monomer and second monomer. The mixed first monomer andsecond monomer are solvent-less vapor deposited onto the membranesubstrate in the deposition chamber. The membrane substrate and themixed first monomer and second monomer are heated to produce in-situpolymerization and provide the composite membrane.

In one embodiment first monomer is dianhydride and the second monomer isdiamine. In one embodiment the heating of the membrane substrate and themixed first monomer and second monomer deposited on the membranesubstrate is performed in the deposition chamber. In another embodimentthe heating of the membrane substrate and the mixed first monomer andsecond monomer deposited on the membrane substrate is performed outsidethe deposition chamber.

In one embodiment the membrane substrate has a first side and a secondside and the solvent-less vapor depositing the mixed first monomer andsecond monomer onto the membrane substrate in the deposition chamberdeposits the mixed first monomer and second monomer onto the first sideof the membrane substrate. In one embodiment the membrane substrate hasa first side and a second side and the solvent-less vapor depositing themixed first monomer and second monomer onto the membrane substrate inthe deposition chamber deposits the mixed first monomer and secondmonomer onto the first side and the second side of the membranesubstrate.

The present invention also provides an apparatus for fabricating acomposite membrane using solvent-less vapor deposition followed byin-situ polymerization on a membrane substrate. The apparatus comprisesa source of a first monomer, a source of a second monomer, a depositionchamber, a mixing chamber in the deposition chamber for mixing the firstmonomer and the second monomer and directing the mixed first monomer andsecond monomer onto the membrane substrate, and a heater for heating themixed first monomer and second monomer and the membrane substrate.

In one embodiment the source of a first monomer is a source ofdianhydride. In another embodiment the source of a second monomer is asource of diamine. In one embodiment the heater for heating the mixedfirst monomer and second monomer and the membrane substrate is locatedin the deposition chamber. In another embodiment the heater for heatingthe mixed first monomer and second monomer and the membrane substrate islocated outside the deposition chamber.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIG. 1 illustrates one embodiment of a system constructed in accordancewith the present invention.

FIG. 2 illustrates another embodiment of a system constructed inaccordance with the present invention.

FIG. 3 shows an embodiment of the source of a first monomer, the sourceof a second monomer, and the mixing chamber system for the preparationof membranes using Solvent-Less vapor deposition followed by In-situPolymerization (SLIP).

FIG. 4 is a graph showing the separation of CO₂ from N₂.

FIG. 5 is a graph showing permeability and selectivity amplificationfactors as a function of film thickness per side.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

Referring now to the drawings and in particular to FIG. 1, oneembodiment of a system constructed in accordance with the presentinvention is illustrated. The system is designated generally by thereference numeral 100. The system is a system for the preparation ofmembranes using Solvent-Less vapor deposition followed by In-situPolymerization (SLIP). FIG. 1 is a schematic of the SLIP process used tofabricate composite membranes.

Some of the structural elements of the system 100 are deposition chamber101, mixing chamber 104, and heater 109. The heater 109 has a heatingelement 110. A power source provides power to the heater 109 throughelectrical connectors 112.

The preparation of membranes using solvent-less vapor depositionfollowed by in-situ polymerization system 100 is performed in thedeposition chamber 101. Monomers dianhydride 102 and diamine 103 areinjected into a mixing chamber 104. The Monomers dianhydride 102 anddiamine 103 are then vapor deposited on substrate 107 as indicated bythe arrows 105 for diamine and the arrows 106 for dianhydride. Thedeposited layer 108 is heated causing polyamic acid to form a polyimide.Polymerization of the deposited layer 108 occurs on the substrate 107(in-situ) providing the membrane. Deposition occurs by a chemical vapordeposition process. No solvents are required for processing at anystage.

The solvent-less vapor deposition followed by In-Situ Polymerization(SLIP) system 100 offers a unique method to fabricate membranes. Thesystem 100 reduces the number of steps involved in membrane fabrication,i.e., the polymer is directly polymerized in the solid state directlyonto the other component of the composite membrane, as compared topolymerizing in solution and then casting onto the component. Unliketraditional membrane fabrication techniques, it does not require theformation of an asymmetric film (typically formed through the use of asolvent or the use of a solvent/non-solvent combination). As a result,there is no need to handle and dispose of solvents as well as removeresidual solvent from the final film. The system 100 can be used tofabricate unique composite membranes that are very thin (less than 400nm). These composite membranes exhibit properties that are an attractivecombination of permeability and selectivity for gas separationapplications.

Referring again to FIG. 1, the solvent-less vapor deposition followed byIn-Situ Polymerization (SLIP) system 100 will be described in greaterdetail. Monomers (dianhydride 102 and diamine 103) or in other wordsprecursors for the polyimide are vapor deposited onto the surface of thesubstrate 107 using the mixing nozzle 104. The monomers 102 and 103 aresimultaneously deposited onto the substrate 107 and polymerize at thesurface of the substrate (i.e., polymerize in situ) to form a polyamicacid deposited layer 108. The thickness of the coating 108 is adjustedby adjusting the deposition temperatures and the length of time thesubstrate is exposed. The coated substrate comprising substrate 107 anddeposited layer 108 are then heated to cause the polyamic acid to form apolyimide. This heating step can either occur in the deposition chamber101, or the sample can be removed from the chamber and heatedseparately. No solvents are required for processing at any stage.

The system 100 enables the design of membranes with improved transportproperties that are better than either the substrate alone or thecoating alone. For example, one approach is to use a substrate that hashigh permeabilities. This substrate is then coated with a thin layer ofpolyimides on one or both sides. The polyimides have been shown toexhibit high selectivities for gas separations. The thin polyimidecoating is used to separate the gases, while the underlying substrateprovides high permeability as well as good structural support. Thesubstrate can be either a dense film or be microporous in nature. Oneexample of a thin dense substrate that meets these requirements isperfluorodioxole.

Another use of the system 100 is to produce a porous substrate. If thepores of the substrate are small enough, the polyimide film can beapplied to one or both sides and the film will bridge the pores. If thecoating does not bridge the pores, techniques commonly used in thesemiconductor industry could be employed to build up successive layersof SLIP coatings. These built up layers can then be designed to bridgethe pores of the substrate.

Another use of the system 100 is to produce a substrate which isinitially dense, and then later form pores in the substrate. Forexample, a dense sheet of ion tracked polycarbonate can be coated on oneor both sides with a SLIP based coating. The coated sample is thenexposed to an etchant in order to remove substrate material and createpores. In this case, the ion tracking in the substrate provides forchannels that can later be etched out to create pores. The substrate isetched to form pores, but a thin film of the SLIP coating remains intacton one of the sides.

An advantages of fabricating membranes using the system 100 is that theproperties of the final membrane (selectivity and permeability) can besimply adjusted by adjusting the thickness of the selective coating.Increasing the thickness of the SLIP coating produces a membrane thathas higher selectivity and lower permeability. Decreasing the thicknessof the SLIP coating produces a membrane that has higher permeability andlower selectivity. This provides a key variable that can be readilyadjusted to design membranes for specific applications.

Referring now to the drawings and in particular to FIG. 2, anotherembodiment of a system constructed in accordance with the presentinvention is illustrated. The system is designated generally by thereference numeral 200. The system is a system for the preparation ofmembranes using Solvent-Less vapor deposition followed by In-situPolymerization (SLIP). FIG. 2 is a schematic of the SLIP process used tofabricate composite membranes. The system 200 provides deposited layers208 and 208A on both sides of a substrate 207 to form a membrane.

Some of the structural elements of the system 200 are two depositionchambers 201 and 201A, two mixing chambers 204 and 204A, and heater 209.The heater 209 has a heating element 210. A power source provides powerto the heater 209 through an electrical connector 212.

The preparation of a membrane using the solvent-less vapor depositionfollowed by in-situ polymerization system 200 is performed in the twodeposition chambers 201 and 201A. Monomers dianhydride 202 and 202A anddiamine 203 and 203A are injected into the two mixing chambers 204 and204A respectively. The Monomers dianhydride 202 and 202A and diamine 203and 203A are then vapor deposited on the two sides of the substrate 207as indicated by the arrows 205 and 205A for diamine and the arrows 206and 206A for dianhydride. The deposited layers 208 and 208A are heatedcausing polyamic acid to form a polyimide. Polymerization of thedeposited layers 208 and 208A occurs on the substrate 207 (in-situ)providing the membrane. Deposition occurs by a chemical vapor depositionprocess. No solvents are required for processing at any stage.

The solvent-less vapor deposition followed by In-Situ Polymerization(SLIP) system 200 offers a unique method to fabricate membranes. Thesystem 200 reduces the number of steps involved in membrane fabrication,i.e., the polymer is directly polymerized in the solid state directlyonto the other component of the composite membrane, as compared topolymerizing in solution and then casting onto the component. Unliketraditional membrane fabrication techniques, it does not require theformation of an asymmetric film (typically formed through the use of asolvent or the use of a solvent/non-solvent combination). As a result,there is no need to handle and dispose of solvents as well as removeresidual solvent from the final film. The system 200 can be used tofabricate unique composite membranes that are very thin (less than 400nm). These composite membranes exhibit properties that are an attractivecombination of permeability and selectivity for gas separationapplications.

The system 200 enables the design of membranes with improved transportproperties that are better than either the substrate alone or thecoating alone. For example, one approach is to use a substrate that hashigh permeabilities. This substrate is then coated with a thin layer ofpolyimides on both sides. The polyimides have been shown to exhibit highselectivities for gas separations. The thin polyimide coating is used toseparate the gases, while the underlying substrate provides highpermeability as well as good structural support. The substrate can beeither a dense film or be microporous in nature. One example of a thindense substrate that meets these requirements is perfluorodioxole.

Another use of the system 200 is to produce a porous substrate. If thepores of the substrate are small enough, the polyimide film can beapplied to one or both sides and the film will bridge the pores. If thecoating does not bridge the pores, techniques commonly used in thesemiconductor industry could be employed to build up successive layersof SLIP coatings. These built up layers can then be designed to bridgethe pores of the substrate.

Another use of the system 200 is to produce a substrate which isinitially dense, and then later form pores in the substrate. Forexample, a dense sheet of ion tracked polycarbonate can be coated onboth sides with a SLIP based coating. The coated sample is then exposedto an etchant in order to remove substrate material and create pores. Inthis case, the ion tracking in the substrate provides for channels thatcan later be etched out to create pores. The substrate is etched to formpores, but a thin film of the SLIP coating remains intact on one of thesides.

An advantage of fabricating membranes using the system 200 is that theproperties of the final membrane (selectivity and permeability) can besimply adjusted by adjusting the thickness of the selective coating.Increasing the thickness of the SLIP coating produces a membrane thathas higher selectivity and lower permeability. Decreasing the thicknessof the SLIP coating produces a membrane that has higher permeability andlower selectivity. This provides a key variable that can be readilyadjusted to design membranes for specific applications.

Referring now to FIG. 3, an embodiment of the source of a first monomer,the source of a second monomer, and the mixing chamber system for thepreparation of membranes using Solvent-Less vapor deposition followed byIn-situ Polymerization (SLIP) is shown. The embodiment is designatedgenerally by the reference numeral 300. The source of a first monomer301, the source of a second monomer 302, and the mixing chamber 306 areconnected so as to mixing said first monomer and said second monomer anddirect the mixed first monomer 303 and second monomer 304 onto themembrane substrate.

The source of a first monomer 302 is an example of a system that can beused as the source of monomers dianhydride 102, 202, and 202Aillustrated in FIGS. 1 and 2. The source of a second monomer 303 is anexample of a system that can be used as the source of monomer diamine103, 203, and 203A illustrated in FIGS. 1 and 2. The mixing chamber 306is an example of a mixing chamber that can be used as the mixingchambers 104, 204, and 204A illustrated in FIGS. 1 and 2.

The source of a first monomer 301 and the source of a second monomer 302are close nozzle evaporators. The first monomer 303 and second monomer304 are under vacuum in the source of a first monomer 301 and the sourceof a second monomer 302. The first monomer 303 and second monomer 304are injected into the mixing chamber 306 and are vapor deposited ontothe surface of the substrate using the mixing nozzle 305. This isillustrated by the arrows 307 representing the first monomer 303 and thearrows 308 representing the second monomer 304.

Applicants are conducting investigation, analysis, and research indeveloping different aspects of the present invention. Theinvestigation, analysis, and research and some of the results of theinvestigation, analysis, and research being conducted by Applicants willnow be described. Perfluorodioxole films (approximately 20 microns inthickness) are being used as substrates for the SLIP process such asthat illustrated in FIGS. 1 and 2. The dianhydride being used for thisprocess is pyromellitic dianhydride (PMDA), while the diamine used isoxydianiline (ODA). Dianhydride and diamine are being deposited on bothsides of the perfluorodioxole substrate, then the coating and substrateis heated to 180° C. for 6 hours. The final polyimide produced by thisreaction of the dianhydride and diamine is poly[N,N′-(phenoxyphenyl)-pyromellitimide] (i.e., PMDA-ODA). The thicknessof the coating is varied by altering the time of deposition as shown inTable 1. Table 1 shows approximate deposition time and nominal thicknessof coating.

TABLE 1 Deposition time per side (secs) in SLIP Thickness (nm) per sideof SLIP apparatus coating 9 100 18 200 36 400

The final membrane is then exposed to a number of individual gases in astandard gas permeation apparatus. The permeability of the film to eachgas can then be calculated. The selectivity of the membrane for a givengas pair can then be calculated by taking the ratio of thepermeabilities. The data is summarized in Table 2. Table 2 showspermeabilities (in Barrers) and selectivities (Ratio of Permeabilities).

TABLE 2 Selectivity N2 CO2 CO2/N2 Perfluorodioxole 516 2934 5.69PMDA-ODA [data from reference 20] 0.2 5 25.0 SLIP 400 nm of PMDA-ODAcoating per 1.29 31.6 24.5 side on Perfluorodioxole SLIP 200 nm ofPMDA-ODA coating per 1.96 34.9 17.8 side on Perfluorodioxole SLIP 100 nmPMDA-ODA coating per 5.22 75 14.4 side on Perfluorodioxole

This data demonstrates that the system 10 enables a membrane to betailored to the desired permeability and selectivity combination simplyby varying the thickness of the SLIP coating. The combination ofselectivity and permeability selected depends on the application. Thisis best illustrated by the data for the separation of CO₂ from N₂ shownin FIG. 4. FIG. 4 shows the influence of thickness of PMDA-ODA coatingper side on CO₂ Permeability and CO₂/N₂ Selectivity.

An amplification factor can be defined to illustrate how the SLIP coatedmembranes exhibit improved permeabilities and selectivities relative tothe coating alone and the substrate alone. For the separation of CO₂from N₂ an amplification factor can be defined for the CO₂permeabilities of the SLIP based membranes relative to the PMDA-ODAusing the following relationship:

${{Permeability}\mspace{14mu}{Amplification}\mspace{14mu}{Factor}} = \frac{{CO}_{2}\mspace{14mu}{permeabilities}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{SLIP}\mspace{14mu}{based}\mspace{14mu}{membranes}}{{CO}_{2}\mspace{14mu}{permeabilities}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{PMDA}\text{-}{ODA}\mspace{14mu}{film}}$

Similarly, an amplification factor for CO₂/N₂ selectivity can be definedrelative to the Perfluorodioxole substrate:

${{Selectivity}\mspace{14mu}{Amplification}\mspace{14mu}{Factor}} = \frac{{{CO}_{2}/N_{2}}\mspace{14mu}{selectivity}\mspace{11mu}{of}\mspace{14mu}{the}\mspace{14mu}{SLIP}\mspace{14mu}{based}\mspace{14mu}{membranes}}{{{CO}_{2}/N_{2}}\mspace{14mu}{selectivity}\mspace{11mu}{of}\mspace{14mu}{the}\mspace{14mu}{Perfluorodioxole}\mspace{14mu}{film}}$

The permeability and selectivity amplification factors as a function offilm thickness per side are plotted in FIG. 5. FIG. 5 shows theamplification Factor for SLIP based films of PMDA-ODA onPerfluorodioxole as a function of film thickness.

The system 10 offers over the following advantages:

Composite membranes fabricated using the system 10 offer the ability toovercome the selectivity and productivity tradeoff WITHOUT the need toform asymmetric membranes.

The system 10 does NOT require solvents, non-solvents, or coagulationbaths. This simplifies the process and eliminates the handling ofhazardous waste.

The system 10 is more flexible than the traditional asymmetric process.It does NOT require the formation of porous support structure based onthe diffusion of solvents and non-solvents.

The system 10 does NOT require that the polyimide be soluble. Thisprovides a great deal of flexibility in material selection.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. The method of fabricating a composite membrane from a membranesubstrate using solvent-less vapor deposition followed by in-situpolymerization, comprising the steps of: providing a vacuum depositionchamber, providing a separate mixing chamber within said vacuumdeposition chamber, injecting a first monomer into said separate mixingchamber within said vacuum deposition chamber, and injecting a secondmonomer into said separate mixing chamber within said vacuum depositionchamber, mixing said first monomer and said second monomer within saidseparate mixing chamber providing a mixed first monomer and secondmonomer in said vacuum deposition chamber, wherein said step ofsolvent-less vapor depositing said mixed first monomer and secondmonomer onto the membrane substrate in said separate mixing chamberwithin said vacuum deposition chamber deposits said mixed first monomerand second monomer onto a thin dense perfluorodioxole substrate,solvent-less vapor depositing said mixed first monomer and secondmonomer onto the membrane substrate in said vacuum deposition chamber,and heating the membrane substrate and said mixed first monomer andsecond monomer deposited on the membrane substrate.
 2. The method offabricating a composite membrane from a membrane substrate usingsolvent-less vapor deposition followed by in-situ polymerization ofclaim 1 wherein said step of solvent-less vapor depositing said mixedfirst monomer and second monomer onto the membrane substrate in saidseparate mixing chamber within said vacuum deposition chamber depositssaid mixed first monomer and second monomer onto a microporoussubstrate.
 3. The method of fabricating a composite membrane from amembrane substrate using solvent-less vapor deposition followed byin-situ polymerization of claim 1 wherein said step of solvent-lessvapor depositing said mixed first monomer and second monomer onto themembrane substrate in said separate mixing chamber within said vacuumdeposition chamber deposits said mixed first monomer and second monomeronto a microporous substrate having pores and said mixed first monomerand second monomer bridges said pores.
 4. The method of fabricating acomposite membrane from a membrane substrate using solvent-less vapordeposition followed by in-situ polymerization of claim 1 wherein saidstep of heating the membrane substrate and said mixed first monomer andsecond monomer deposited on the membrane substrate heats the membranesubstrate and said mixed first monomer and second monomer deposited onthe membrane substrate to less than 180° C.
 5. The method of fabricatinga composite membrane from a membrane substrate using solvent-less vapordeposition followed by in-situ polymerization of claim 1 wherein saidstep of heating the membrane substrate and said mixed first monomer andsecond monomer deposited on the membrane substrate heats the membranesubstrate and said mixed first monomer and second monomer deposited onthe membrane substrate to less than substantially 100° C.